This June issue of Walking Your Fields newsletter contains articles about: corn stand evaluation, planting date impact on soybeans, nitrogen loss after rain, and early weed control in corn. Articles are written by DuPont Pioneer agronomists in Minnesota and Wisconsin and are distributed on behalf of DuPont Pioneer account managers and Pioneer sales reps.

1. A late spring and wet conditions have delayed soybean
planting in some areas. This article addresses how later
planting affects soybean development and maturity.
Soybeans are ‘day length sensitive’ plants. This means
flowering and maturity are triggered by the length of the
night or dark period. When adapted maturity varieties are
planted before the middle of June, the flowering is trig-
gered by shorter days (actually longer nights) after June
21. For each three to five day delay in planting, flow-
ering and maturity are delayed only about one day.
That means that if you plant the same variety on May 10
and June 10 (30 days later), the flowering and maturity of
the later planting is delayed about six to ten days. This
happens regardless of the maturity of the variety.
Soybean Development. Flowering usually begins six to
eight weeks after seedling emergence and continues for
three to four weeks. However, flowering will occur earlier
if soybeans are planted late. Warm temperatures acceler-
ate development, especially flowering. A full season soy-
bean variety will normally flower in the first ten days of
July. If there is a hot period in mid June, it can flower up
to two weeks earlier. When this happens, and if there is
good weather for the rest of the season, higher yields
result because of the extended reproductive period. The
soybean plant may flower ten days earlier, but it won’t
necessarily mature ten days earlier because of day length
controls. Generally, yield increases as the length of the
flowering to maturity stage increases.
Warm weather in August or
early September does not
hasten maturity much, un-
less it causes water deficit
stress. Sometimes, temper-
ature is blamed for stress,
because hot temperatures
often accompany drought. A
cool fall does not delay ma-
turity much either, although
yields may be reduced if the
cool weather is accompa-
nied by heavy rain, causing
disease. The flowering peri-
od is influenced more by
temperature, but maturity is
more strongly influenced by
photoperiod. The rapid
shortening of days starting in mid August drives the soy-
bean to maturity; temperature has only a small influence
on maturity.
The average yield of corn in the U.S. and Canada has
tripled over the last half-century. Yield gains have result-
ed from improved hybrid genetics, more precise soil fertil-
ity practices, better weed control, and advances in other
production methods. Among these factors, genetic im-
provements have contributed the most to yield gains,
adding from 1.0 to 1.5 bu/acre each year. To accomplish
these increases, corn breeders have selected for superior
tolerance to drought and other stresses, and yield stability
across diverse growing environments. A key result of en-
hanced stress tolerance is adaptability of hybrids to high-
er plant populations. Optimum economic returns often
require plant populations of 35,000 plants/acres or more,
depending on the hybrid and environment. Obtaining the-
se higher plant populations is essential, especially at
higher yield levels.
It is important to evaluate stands shortly after emergence.
Corn stands can be reduced by many issues such as cold
or wet soils, insect feeding, poor seedbed, and equipment
issues. Shortly after emergence is the best time to evalu-
ate stands and the effectiveness of planter equipment,
residue management, and seedbed preparation.
Jerome Lensing
Product Agronomist
Ryan Bates
Field Agronomist
Brian Buck
Field Agronomist
June 10, 2013 - Issue 3
Planting Date Impact on Soybean
Development and Maturity
Photo: Pat Branick, DuPont Pioneer
Photo:TomDoerge,DuPontPioneer
Evaluate Corn Stands: Identify Areas
for Improvement
Continued on pg 2
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2. Stand counts. Take several sample counts to represent
the field or area under consideration. For ease of calcula-
tion, a sample size of 1/1000th of an acre is recommend-
ed (Table 1). Measure off the distance appropriate for
your row width, count the number of live plants and multi-
ply by 1000 to obtain a reasonable estimate of plants/
acre. Stand counts should be taken randomly across the
entire area of the field. The accuracy of your stand esti-
mate will improve with the number of locations sampled
within the damaged area.
Stand uniformity. Evaluate
whether plants are evenly
distributed or if the stand is
uneven. Gaps and doubles
indicate an equipment or
seedbed problem and re-
duce yield.
Identify Problems and
Plan for Solutions. After
you have evaluated stands
and uniformity, make every
effort to identify the causes
of any observed problems.
Were metering devices
working properly? Were row
cleaners set properly and working effectively? Was plant-
ing depth proper and consistent? Was residue managed
effectively? Were monitors effective in identifying meter-
ing device or other equipment problems when they oc-
curred? Was pre-plant tillage uniform and did it create a
quality seedbed? Continuously improve your planting op-
eration by making sure to implement solutions to any
problems observed.
Heavy spring rainfall causes concerns about nitrogen (N)
losses in corn fields and raises questions about the need
for supplemental N applications. There are two types of N
loss: 1) Leaching—movement with water below the root
zone, and 2) Denitrification—loss to the atmosphere. In
general, leaching losses are more likely on sandy soils
where water can move through the profile quickly. Denitri-
fication is more likely on medium and fine textured soils
that are not well drained. The exact extent of N losses
through leaching and/or denitrification following the heavy
rains is difficult to determine. Both of these loss process-
es occur through the nitrate form of N, so the potential for
significant loss is determined by the amount of the N sup-
ply that was in the nitrate form when the excess rainfall
occurred. Losses depend on many factors such as the
time of N application, the forms of N applied or expected
to provide N for the crop, soil characteristics, and soil
moisture.
Where fertilizer N was applied before planting, the timing
of the application and the form of N used are important in
determining the risk of loss. Keeping in mind that losses
occur through the nitrate form of N, the timing of nitrate
formation is an important consideration. Fall-applied ferti-
lizer N has a high risk for loss following excess rainfall
because most or all of the N would be in the nitrate form
by mid-May. For spring preplant applications, ammonium
forms of N such as anhydrous ammonia or urea are con-
verted to nitrate-N in about four to six weeks. Urea usual-
ly is converted to nitrate more rapidly than anhydrous am-
monia. Nitrogen solutions (28% UAN) contain half of the
N as urea and the remainder as ammonium nitrate. Es-
sentially, this fertilizer contains 75 percent of the N as
ammonium and 25 percent as nitrate when it is applied.
Denitrification losses can occur within a few days, if the
soil remains saturated or flooded and nitrate-N is present.
Warm temperatures and extended periods of saturated
conditions favor high losses. Work in Illinois suggests that
four to five percent of the nitrate-N present can be lost
each day the soil remains saturated. Table 2 from the
University of Nebraska provides some estimates of deni-
trification losses at various temperatures and times of
saturated soil conditions.
Options for applying supplemental N when it is needed
include traditional side-dressing with anhydrous ammonia
or N solutions. Where the entire crop N requirement has
not yet been applied, side-dress or other post-emergence
applications should contain the balance of the crop N re-
quirement plus 25-50 percent of the N that was already
applied. Urea-ammonium nitrate solutions (28%) can also
Table 1. Row lengths
equal to 1/1000th of an
acre.
Row Width
Length of
Row
38 inches 13 ft. 9 in.
36 inches 14 ft. 6 in.
30 inches 17 ft. 5 in.
22 inches 23 ft. 9 in.
20 inches 26 ft. 2 in.
15 inches 34 ft. 10 in.
Photo:TomDoerge,DuPontPioneer
N Loss Following Excessive Rainfall
Continued from pg 1
Table 2. Estimated denitrification N losses influ-
enced by soil temperature and days saturated.
Soil Temp
(°F)
Days
Saturated
N Loss
(% of applied)
55-60
5 10
10 25
3 60
75-80
5 75
7 85
9 95
Source: Shapiro, University of Nebraska.

3. be applied as a surface band or as a broadcast spray
over the growing crop. Dry N fertilizers such as urea or
ammonium nitrate can also be broadcast applied to the
crop. Leaf burning from solution or dry broadcast applica-
tions should be expected. Applying the dry materials
when foliage is dry will help to minimize burning. Basical-
ly, broadcast N rates should be limited to 90 lbs. N/acre
for corn with four to five leaves and to 60 lbs. N/ acre for
corn at the 8-leaf stage. Under N deficient conditions,
corn will respond to supplemental N applications through
the tassel stage of development if the N can be applied.
To help you assess potential fertilizer N losses, download
the simple and reliable Supplemental Fertilizer N Work-
sheet from the University of Minnesota. http://
www.joe.org/joe/2005june/tt4.php (Source: Schmitt and
Randall, U of MN)
(Source: Dr. Larry G. Bundy, U of WI Extension Service.
‘Evaluating Nitrogen Losses Following Excessive Rainfall.’)
Application timing is everything when only using glypho-
sate to control weeds in corn, as it has no soil residual
activity. The weeds need to be emerged, but not too large
that they compete with corn and reduce yields. Usually
this control is accomplished with multiple applications of
glyphosate, and/or by using glyphosate in combination
with herbicides that provide residual weed control.
Critical Weed-Free Period. The critical period of weed
control is the interval when weed control is required to
provide maximum yield. Weed competition before this
period will not affect yield, if weeds are controlled by the
start of the critical period. Weed competition after the criti-
cal period will not affect yield.
Figure 1 shows weeds removed in the first four weeks
after planting, corn yield was 100 percent (solid line).
When the crop was kept weed-free for at least seven
weeks, corn yield was also 100 percent (dashed line).
It is difficult to predict the critical period, because it de-
pends upon the weeds (how competitive the different
weed species are, their density, when they emerge), and
the environment (soil moisture and nitrogen). Research
conducted on weed interference in corn indicates that
weeds should be controlled by at least the third week
after planting, and weed emergence should subsequent-
ly be prevented until about eight weeks after planting (or
more accurately, 14-leaf or V12 corn). If a non-residual
herbicide like glyphosate is applied three weeks after
planting, a second herbicide application or cultivation will
probably be required to keep later germinating weeds
under control, or a residual herbicide will need to be add-
ed to prevent further weed germination during this win-
dow.
Herbicide Systems. According to research by University
of MN, Dr. Jeffrey Gunsolus, the longer the duration of
weed competition, the greater the impact on yield. His
research concluded:
 The best economic return came from a pre-
emergence application of acetochlor followed by a
post-emergence application of glyphosate at the 5-
inch weed stage.
 A one-pass application of acetochlor plus glyphosate
at the 1-inch weed stage can maximize yields.
 One post-emergence application of glyphosate did
not maximize yields or returns in his study (Gunsolus,
2005).
Always read and follow label directions and safety pre-
cautions.
NCGA National Corn Yield Contest
This NCGA contest has given participants the recogni-
tion they deserve as well as an opportunity to learn from
their peers.
 June 15: Early entry deadline. Cost per entry $80.
 July 12: Final deadline to have all entries submitted.
Cost per entry $110
Do you have a nice looking field of Pioneer corn?
Enter a Pioneer®
brand hybrid in the yield contest and all
entry fees and membership dues are paid by Pioneer
directly to NCGA.
Talk to your Pioneer Sales Representative about
your entry.
Online entries are available by following this link:
https://membership.ncga.com/CornyieldContest/
Early Weed Control is Key in Corn
Figure 1. Source: Reprinted with permission from Chris
Boerboom, U of WI. ‘Timing Post-emergence Herbicides
in Corn and Soybeans.’

4. WALKINGYOURFIELDS®
KC Bates, Buck, Lensing
DuPont Pioneer
Sales & Marketing
PO Box 466
Johnston, IA 50131
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and DuPont Pioneer Account Manager.
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In this issue:
 Planting Date Impact on
Soybeans
 Corn Stand Evaluation
 Nitrogen Losses Following
Excessive Rainfall
 Early Weed Control in Corn
 NCGA National Corn Yield
Contest